1. Field of the Invention
The present invention relates generally to plating baths and methods for monitoring the quality of the bath. More particularly, the present invention relates to an alternating current (ac) voltammetric or tensametric method that provides detailed ac current spectra or fingerprints which have unique patterns that are related to and indicative of those trace constituents in the plating bath which are known to affect plating quality of the plating baths. Use of the ac current fingerprints provides a valuable method for monitoring changes in the trace constituent make up of plating baths and evaluating plating bath quality.
2. Description of the Background Art
The important properties and characteristics of plating deposits include tensile strength, ductility, solderability, uniformity, brightness and resistance to thermal shock. These plating deposit properties are largely controlled by minor quantities of organic addition agents, their degradation products, chemical contaminants and other trace constituents which affect plating.
It is important that the plating bath be continually monitored to insure that concentrations of the trace constituents remain within limits required to achieve the desired plating deposit. The accurate control of plating deposits is especially important in the plating of copper on printed circuit boards and other electronic circuit components where the quality of the copper plating must be closely controlled. Accordingly, there has been a great deal of interest in developing methods for monitoring and controlling the levels of trace constituents in copper plating baths.
The methods presently used to monitor trace constituents in plating bath solutions include differential pulse polargraphy, cyclic voltammetric stripping, high performance liquid chromatography (HPLC) and UV fluorescence. None of these techniques has been entirely successful for a number of different reasons.
One of these methods which has been used to evaluate plating bath solutions is based on the use of cyclic voltammetry. The method is set forth in U.S. Pat. No. 4,132,605 and basically involves using a dc function generator to sweep a working electrode positioned in the bath solution through a voltammetric cycle including a metal plating range and a metal stripping range. A counter electrode is also positioned in the bath solution in series with the function generator and a coulometer is used to measure the charge of the metal stripping portion of the cycle.
The cyclic voltammetric method provides a single indirect signal which is related to the plating rate and which is useful in monitoring plating bath solutions with one or two trace constituents that affect plating rates. However, not all trace constituents and addition agents which affect plating properties also affect plating rates. Also, the addition agents for many plating baths contain two, three or four active trace constituents. Each of these active trace constituents exerts a different effect on the properties of the plated deposit. Further, the effect on plating properties of a certain concentration of these various constituents is modified by interaction with trace metal ion concentrations, trace anion concentrations and aging of the bath. Aging of the bath results in the accumulation of trace impurities and degradation products which interact and affect action of the addition agents. The numerous trace constituents present in many plating baths and the extremely complex interactions between these constituents which determines final plating deposit characteristics make the single response signal of the cyclic voltammetric technique less than adequate when close control and monitoring of the plating bath is required.
Accordingly, there is a present need to provide a simple and effective method for monitoring high quality plating baths which provides more complete and meaningful information regarding the effective concentrations of trace constituents in the plating bath. Such a method is required to provide the accurate control of plating deposit properties which is necessary for high quality plating operations.